1. Field of the Invention
The present invention relates to a hydraulic control system, and more particularly to a hydraulic control system wherein a plurality of shift valves are coupled with a manual valve such that both of the shift valves are always supplied with a line pressure as long as the manual valve is set in an automatic forward drive range.
2. Description of the Prior Art
Automatic transmissions may be classified into two groups from the standpoint of arrangement of shift valves with respect to a manual valve. One group employs a so-called "series arrangement" wherein a 2-3 shift valve is coupled with an outlet port of a 1-2 shift valve which is coupled with a single outlet port of a manual valve such that the 2-3 shift valve is supplied with a line pressure only when the 1-2 shift valve is in its upshift state. The other group employs a so-called "parallel arrangement" wherein a 2-3 shift valve is coupled with an outlet port of a manual valve different from or the same as that outlet port of the manual valve with which a 1-2 shift valve is coupled.
A known control system for an automatic transmission disclosed in U.S. Pat. No. 3,667,323 issued to Namio IRIE on June 6, 1972 and assigned to Nissan Motor Company, Limited falls into the second group. According to this known control system, a 2-3 shift valve has an inlet port communicating with an outlet port of a manual valve via a conduit provided with a unitary structure including an orifice and an one-way check valve and, in operation, this conduit serves not only as a pressure supply passage but also as a drain passage. Upon making an upshift from a second gear ratio to a third gear ratio during operation when the manual valve is set is D range, the line pressure always supplied from the manual valve to the 2-3 shift valve is allowed to reach friction units necessary for establishing the third gear ratio. During this transitional, period the orifice of the unitary structure plays a role in controlling the rate of transmission of the oil to those friction units. Upon setting the manual valve to a manual second (II) range from D range when the vehicle is moving with the third gear ratio, an additional bias tending to urge the 2-3 shift valve toward a downshift state against a governor pressure indicative of the vehicle speed is created, and the conduit including the unitary structure is drained off, wherein, as long as the vehicle speed is high enough to hold the 2-3 shift valve in the upshift state, oil is discharged from the friction units through the conduit by opening the one-way check valve of the unitary structure. Therefore, the discharge of the oil from the friction elements is swiftly carried out without any flow restriction of the orifice of the unitary structure. The unitary structure including the orifice and the one-way check valve therefore is indispensable for shockless upshift operation to the third gear ratio and for swift transition from the third gear ratio to the second gear ratio to provide effective engine braking upon setting the manual valve to II range.
This known control system has problems in that troublesome work is necessary for inserting in the conduit the above-mentioned unitary structure including the orifice and one-way check valve, and in that the number of component parts is great, increasing difficulty in controlling the component parts and decreasing efficiency in the assembly work. Another problem is that oil tends to leak past that portion at which the unitary structure is connected to the oil conduit, resulting in a drop in reliability.
Another known control system which falls in the second group is disclosed in U.S. patent application Ser. No. 962,738 filed on Nov. 21, 1978, Kazuyoshi IWANAGA, et al., wherein a 2-3 shift valve is coupled with the same outlet port of a manual valve as that outlet port with which a 1-2 shift valve is coupled. In this known control system, since that conduit through which the pressure is supplied from the manual valve to the 2-3 shift valve is always supplied with the line pressure, this conduit does not serve as a drain conduit. Thus, a II range pressure which appears upon setting the manual valve to II range is exerted on the 2-3 shift valve to urge the same toward a downshift position against governor pressure and the oil is drained via a drain port of the 2-3 shift valve. As a countermeasure against the possibility that, should the 2-3 shift valve become stuck in the upshift state thereof, the transmission would be interlocked upon setting the manual valve to a manual first (I) range, an interlock valve must be provided which is responsive to the engagement of the friction unit to be engaged upon establishing the third gear ratio to prevent the supply of a manual first (I) range pressure to a friction unit to be engaged for establishing the first gear ratio in the I range.
Thus, a problem of this known control system is the need for an additional valve, i.e., the interlock valve.